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    SubjectRe: [patch 2/2] sched: Scale the nohz_tracker logic by making it per NUMA node
    On Mon, 2009-12-14 at 14:32 -0800, Pallipadi, Venkatesh wrote:
    > On Mon, 2009-12-14 at 14:21 -0800, Peter Zijlstra wrote:
    > > On Thu, 2009-12-10 at 17:27 -0800, wrote:
    > > > Having one idle CPU doing the rebalancing for all the idle CPUs in
    > > > nohz mode does not scale well with increasing number of cores and
    > > > sockets. Make the nohz_tracker per NUMA node. This results in multiple
    > > > idle load balancing happening at NUMA node level and idle load balancer
    > > > only does the rebalance domain among all the other nohz CPUs in that
    > > > NUMA node.
    > > >
    > > > This addresses the below problem with the current nohz ilb logic
    > > > * The lone balancer may end up spending a lot of time doing the
    > > > * balancing on
    > > > behalf of nohz CPUs, especially with increasing number of sockets and
    > > > cores in the platform.
    > >
    > > If the purpose is to keep sockets idle, doing things per node doesn't
    > > seem like a fine plan, since we're having nodes <= socket machines these
    > > days.
    > The idea is to do idle balance only within the nodes.
    > Eg: 4 node (and 4 socket) system with each socket having 4 cores.
    > If there is a single active thread on such a system, say on socket 3.
    > Without this change we have 1 idle load balancer (which may be in socket
    > 0) which has periodic ticks and remaining 14 cores will be tickless.
    > But this one idle load balancer does load balance on behalf of itself +
    > 14 other idle cores.
    > With the change proposed in this patch, we will have 3 completely idle
    > nodes/sockets. We will not do load balance on these cores at all.

    That seems like a behavioural change, not balancing these 3 nodes at all
    could lead to overload scenarios on the one active node, right?

    > Remaining one active socket will have one idle load balancer, which when
    > needed will do idle load balancing on behalf of itself + 2 other idle
    > cores in that socket.

    > If there all sockets have atleast one busy core, then we may have more
    > than one idle load balancer, but each will only do idle load balance on
    > behalf of idle processors in its own node, so total idle load balance
    > will be same as now.

    How about things like Magny-Cours which will have multiple nodes per
    socket, wouldn't that be best served by having the total socket idle,
    instead of just half of it?

     \ /
      Last update: 2009-12-15 00:01    [W:0.022 / U:13.580 seconds]
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